Marine seismometer spread



N. POTTORF MARINE SEISMOMETER SPREAD Sept. 9, 1952 3 Sheets-Sheet 1Filed Feb. 23, 1949 p 9, 1952 N. POTTORF MARINE SEISMOMETER SPREAD 3Sheets-Sheet 2 Filed Feb. 23, 1949 5 Sheets-Sheet 3 N. POTTORF MARINESEISMOMETER SPREAD Sept. 9, 1952 Filed Feb. 25, 1949 "UNITED STATEPatented Sept. 9 1952 ARINE VSEISMOMETER srREAn p I Newell Pottor'f,mulsa, kla.,-assignor-to Stano'lind -0ilrand Gas-Company,

l "tion of Delaware Tulsa, 0kla.,-a' corpora- ApplicationFe'bruary23,1949,'SerialNo;77;726 3 V g 17 "Claims; (01. '1'77 352')'Thisinvention'relates to geophysical surveying "and is "directedparticularly to prospecting bythe seismic method over water-covered-areas such as in the Gulf of'Mexico.

Geophysical surveying using artificially-created seismic waves has beenextensively "and successfully used on landfora'number of yearsybut it isonly comparativelyrecently that *the method has been applied tooff-shore explorations-for oil and gas, particularly "in the Gulf ofMexico. In the earliest applications of the seismic method to marine-=area's, theshots a'nd detectors were individually placed on' or'underthemarine floor in 'muchj-the-same manneras in-prospecting onland.

The results obtained were generally similar to those obtained onlandprospects;

With the improvement and adaptation of specific techniques andinstruments for this marine work, the speed of prospecting- "by thismethodhas increased so markedly-over what was previously possible'eitheron land or water that,

as'a result, more than the normal ratio o'f geophysical effort in marineprospecting has been 'concentratedon the seismic method as compared withthe magnetic and "gravim'e'tri'c methods, for example;

Both now and in the past jone of the diflicult problems connected withthie -method has been the proper handling of--the seism'ometers "usedfor detecting the seismic waves. 'seismometers at known locations in aspread on the marine floor as in landprospecting roved Placing the evenmore laborious and time-consuming than on "land. Towing a spread ofseismometersconneeted together by a conductor and tension-cable alongthe marine floor or supported by; floats behind the recording vesselfrom one location to another, and shooting either with the seismometersonthe marine floor or supported from the floats have resulted in markedincreases in the "speed of carrying out the geophysical surveys.

However, dragging of the seismometer" spread along the seabottompresents disadvantages in the obvious hazards both to the equipment andtothe marine Iife'and to installations located on the marine floor.Employing "seismometers at or near the surface and supportedfby floatsresults in'the picking up of a great deal 'ofnoise,

even under favorable "conditions of low wind velocity and relativelysmooth water surface. Even in "calm seas, the-noise pickedupby'nearsurface seismometers issuchas to maskimost of the-desiredweakreflectionsvvhile on'windy days and when the-water surface is rough,the'no'ise maybe-so strong 'astooVerrideaIl reflections and makeprospecting impossible. As a matter -lof fact, there are some seasons"of-the-year in -the Gulf of -MeXic0 wh'enthe noise conditions-:irom thewater surface have been so-troubl'esome that prospecting operations werepossible-only a sii1a11 fraction" of the time. -The resultant delayswhile thecrews and equipment are held in readiness'ztor favorableworking conditions add vory gr'eatiyito the expense of the operation. eIt is; -'accordingly, a primary object o'f' my invention to'provideaseishric-Witvedettitor spread for marine seismograph prospecting whichgives a greatly improved signal-to-nois ratio per mi'tting detection'o'f cleoper and weaker desired seismic reflection signals. Anotherobje'ct is to provide a marine seismo n'eter spread lin which theoperating depth'is automaticallycontrolled to bring the seismometers tothemosteflective depth for receiving signals A -turther objectds toprovide a marine seismometer spread-which can operate under adverseweather or water conditions and obtain good geophysical data "whichcould not hithertobe obtained under such-eon- "ditions. Stillanotherobject'isto rprovideaitowable marine seismometer spread "whldhitli'e depth of 'submergence of the seismometers"is automaticallyfloat-controlled from' i the water surface, but in a manner whlchmini-mizes tlte transmission of noisesignals'to the-seisniometers.

A still further object is to provide a towable "marine seismometer'spread which creates arlatively small drag on the towing vessel therebyreducing the travel time :betweenshotpoints'aiid increasing the speed-of the prospeotingopera tions. Still another' a'nd further-*objectistofipvide a marine seismometer spread 'having' -good-:- discrimination orfiltering against the -transmi's' ision of vibrations along the towingandconnetmg cable or the suspension "cables 'c'onnec'ti'ng theseismometersi to the float 'supports. Another and =st1=1-I furtherobjector-my 'invention'is a provide a marine seismome'ter spreadcapable- "of use in areaswherethe-depth of water is too great-"to permit"prospectingby methods which require placing the seismometers onthe-marine floor. *Otherandfurther objeets'; uses, and advantages "ofthe invention "will become apparentas the descriptionproceeds; F

*From observations made under a- "variety or conditions, ithas now beenfoundthat' tlierange of depths in water whereseismometersunay' -beplaced for thenmost efiicient operation .is'trela tively narrow. Due tothe greatly ambient seismic-wave transmission propertiesof ai'ri ndwater, seismic"waves"traveling"upward fronr tho quarter of the seismicwave length in that I medium. To avoid this possible interference,

which changes the character or appearance of .the detected-waves, it istherefore desirable to locate the seismometers as close as possible tothe water surface. It is at this surface that the Figure 1 is across-section of a body of water displacements are a maximum and hencevthe.

possible interference is a minimum.

However, as was briefly indicated above it has i H been found that thelevel of the noise within the seismic wave band is a maximum at thewater operation. The upper limit of this range. is imi mediately belowthe zone of surface water noise 1 andisfietermined by the, place wherethe noise .level drops to a satisfactorily low value. The lowerlimit ofthe range is the depth at which -.:mterferenceeffectsbecome pronouncedfor waves of ,lnterest inthe seismic band and is in practice omewhatless than the quarter-wave-length depth,"w,hichiswhere the interferencebecomes a maximum. 1 Partial interference is noticeable atshallowerdepths. This range appears to be be- :tween 5v and 15 feet,with the preferred depth of operation being about 10 feet. Above thisrange the noise level is generally appreciable, while below itinterference is observed for waves of interest in the seismic band. 1

- L, Itis, of course possible at the expense of intro- 1 ducing more orlessnoi e into the recorded signals to. work somewhat above this rangeand in the :lowerwportion of thesurface noise zone, if it is.desirableor necessary to do solfo'r any reason,

suchasin-shallow water. Also it isconceivable that useful results, suchas the emphasis of certain reflections, first arrivals, or the like,might follow fromdeliberately choosing a depth where interdesired depthat a shootinglocation. This depth control is achieved by a particularcombination of fbuoyancies of the cableand seismometer- .supportingfloats, and of the connecting leads or cables between the submergingseismometer floats and the floats at the water surface. Specifically itis preferred that the seismometer-containing floats have asmallpositivebuoyancy, the surface cable-supporting floats have a somewhatlarger positive buoyancy, and the connecting lead or leads between thesurface floats and the seismometer float have a negative buoyancy suchthat a portion of said lead is suflicient to balance the =;positivebuoyancy of the seismometer float. As

a result, when its forward motion is stopped, the

"seismometer float submerges to a depth at which [its positive buoyancyis exactly balanced by the negative buoyancy of part of the connectinglead or leads. This provides an automatic control of the depth andlocation of the seismometer float,

which with the surface float forms a loosely-- coupled'system suchthatnoise vibrations tend to =more detail thelmanner of connection or tsewith my invention;

be filtered out and not transmitted along the supporting lead from thewater surface to the submerged seismometer float.

This will be better understood by reference to the accompanyingdrawings, forming a part of this applicationand illustrating typicalembodi-' ments of my invention, in which drawings like numerals areapplied to ,the same or correspondving parts in the different figures;In these draw-.

ings,

in which ,a marine, seismometer spread constructed in accordance withthe invention is shown in a position for making a record;

Figures 2 and 3 are respectively an elevation and a cross-section viewof a seismometer-supporting-float; V j

Figure 4 is'a cross-section of 'a seismometer mounting and housing;

Figure 5 is a view of a'cable-supporting float;

Figure 6 isa plan .view of a complete spread and auxiliary equipmentoperating in accordance Figure? is a cross-section of a bojdy offwatershowing an alternative embodiment of the inventionsuspended therein;

Figures 8 and9 are elevation views showing, in

ments of the invention;

Figure 10 shows a modification of .the connect-- ing leads; a V,

Figure v11 is a detailed drawing showing a modification of the spread ofFigure 7 and .7 1

, Figure 12, is an elevation v ew showing an alterleads to theseismometer'spread.

native manner of connecting the depth -control Referring now to thesedrawings in d'eta to Figure 1 in particular, a vessel 20 is shownproceeding through a body of water 2l towingfa spread 22 constructed inaccordance with the invention. Spread .22 is made up'of acablej23attached to'vessel 20 havingboth strands withfa considerable tensilestrength for connectingthe various components and-insulated electricalconductors for the seismometer leads. This cable supported at or nearthe water surface by a pluralityof spaced floats 24 attachedto itat-intervals along its length. Also spaced along cable 23, ordinarilyvat somewhat, larger intervals, are a pluralityof seismometer floats 25,each of which has a pair of flexible leads 26 and 2T,attached to itsends and connecting-it to a surface float 28 on the cable 23. Cable 23is maintained in tension to prevent appreciable saggingof theunsupported portions thereof betweenthe floats 24 by a drag or float 29at the trailing end ofthespread, which float may carry a flag 30 toindicate the position of the end ofthe spreadto an observer. Inaccordance with my invention, the buoyancy of each seismometer float 25is slightly positive, while the buoyanc'y'of the supporting fldat 28 issufficieritly positive to keep it always-at .the water surface, and theconnecting leads 26 and 21 are together negatively buoyant by-an amountmore than twice as large as the positive buoyancy of float 25. Thismeans that a. pore tion less than half of the length of eachoflthe leads.26 and 21 is able to and statement exactly balancing the positivebuoyancy of float .25, so that it is held submergedatfsome depth in thewater 2L. Connecting lead-s26 and- 21 thus hang with slack loops whichminimize the transmission or; ,noise or other disturbances from thewater-surface to theseismomter float I 5 a e th leadsfi ce the Sensiengeese ea e240 55 leads is smail'as they do not have .to support theweight of the-seisr'nometer, they are free to move with the wave "motionof-the wat'envand thus create a minimum of. turbulence. and resultantnoise in theaseismicsbanda It will be observed that theposition'o'ccupied by the. float 25 is one of stable equilibrium becauseof the fact that. as it rises, a larger fraction-of the weight-ofconnecting cable leads 26 and-21 opposesthe'motio-n, while, if it sinks,more of the weight :of these loads is transferred from it to the float28. Depth adjustment and leveling of the seismometer float 2-5 istherefore accomplished by regulating the length of each of thesupporting leads 2'6 and 21 which is carried by the surface :float 28.The length of leads 26 and '21 carried float :25 is -"constant as longas its buoyancy does not vary.

One of the seismometer floa't's 25 is shown in more detail in Figuresand 3.. As is shown clearly by Figure '2, the float, constructed of woodor other suitable buoyant material, is preferably elongated andstreamlined by the provision of pointed ends so as'to be easily towablethrough the water. The :buoyancy of'fthe float 25 is adjusted to thedesired-positive 'value by the additionor removal of small-straps oflead "33, which are Wrapped around the cable 26 or 2! Where it emergesfrom the pointed ends of the float. The flo'at 251spreferablyconstructed in two symmetrical "sections or halves adapted tobe fastened together, each half being in the form *of the longitudinalcross section shown in Figure 3, and the interior being provided withthe passages 34 and 35 into which "the connecting leads 26 and 21 enterand are anchored and a central chamber 36 which houses the seismomete'rcase and assembly. Insulated electrical conductors in one or both ofleads 2-6 and 2'! "are utilized to connect a s'eismometer in chamber "36to appropriate insulated "reads in the-cable 2-3. Enlarged openings 3!and 38 to the passages 34 and 35 at each end "of the fioat 25 providespace for resilient sleeves 39 and "40 which help prevent too sharpbending and breakage of the leads 26 and 2'! where they enter the float.Enlarg'em'erits 41 and 4-2-inpassages =34 and 35 surround anchoringclamps which grip and hold the lead-s26 and"2l. 1

In Figu're '4 is shown a "suitableseismometer assembly consisting' of aseism'ometer trunnion mounted in a frame "5l,whichis -set in a pair ofanti-friction bearings 52 and 53. As the center of gravity of thissuspended system, consisting of 'the'frame 5| and seismometer 50, is atthe point :1: below the axis of rotation of the bearings '52 and 53,this acts as a self-righting system to maintain. seismometer 50. alwaysin an. upright (position. The electrical leads from seismometer 50 arebrought out through. the bearing'53 to a pair of slip-rings- 54 and '55,which are contacted by brushes 56 and 5:! connected through either orboth of suspending cables 26 and 2'! to suitable insulated'conductors inthe cable 23 by a waterproofsplice. A watertight, cylindrical housing 58entirely surrounds, this seismoineter assemblyand is -fitted within thecavity 36 in thefloat 2 5.

A typical cable-supporting -fioat 2-4 is hown in Figures. This may be.c-onstructed of. wood like the seisrnometerfloat 25 in two halves whichare clampedtogether urrounding the cable 23, as by means of encirclingmetal bands-60, or otherwise fastened together by bolts onscrews.

The plan view of Figured-illustrates atomsei'smometers.

"6 l the relative positions occupied. during fslzootingoperations, Forsimplicity thecable. fl'oats .24 .and depth-controlling floats 28 havebeen omi-tted fromthis figure, and only the positions ofthe-seismo-meter floats 25 relative to the two vessels are shown. Tenseismom-e'ter's are employed spaced uniformly apart by distances oftheord'e'r of 200 or -250,-feet,except for the two end seismometerswhich are spaced from the adjacent seismometers by distances of 50 to.feet. The'se end seismometer pairs are moreclosely spaced than theothers for the-reason that-if the signal from one is accidentallyobscured by noise, the signal from the other :may be used for obtainingreflection or moveout times.

In a typical method of operation, the vessel 20 and spread 22 areaccompanied by an. explosives-carrying vessel 64 traveling along aparallel course. At the shooting location the explosives vessel '84places an explosive oharge .65 and pays out a firing line BB. Thelocation of charge 65, which may be suitably supported by floats or inany other manner either above or :below the water surface, is normallyoffset from the line of spread 2'2 'by'the distancedof the order of 300feet.

In timed relation to the placing of the charge 65, the forward progressof the vessel 20 and or the spread 22 is slowed down and stopped so thatthe center of spread 22 is directly opposite the charge 35. As soon asthe for'wardmotion of the spread-has substantially ceased, theseismometers which, due to their small but finite drag, stream back-wardfrom the supporting floats 28 and run near the water surface duringtowing, are pulledd-own by the non-buoyant cables 26 and 21 and come toequilibrium at the desired depth and location in the water. When thisdepth is reached, after a period of l to '3 minutes, the charge 65 isdetonated,'and the-record is made. As soon as the record is taken, thevessel 20 immediately resumes towing of the spread 22, and theseismometers, streamingback wardly'fr'omthe supporting 'floats- 28,rise-up and tow near the surface while traveling to the next shootinglocation.

In the embodiment of the invention illustrated in Figure 7, theseismometer floats 25 "are towed directly from vessel 20 rather thanthrough the supporting floats 28 in such a manner that-they tow directlyunder floats '28 and remain near the desired shooting depth duringforward motion of the vessel 2'll. Thus the cab1e'2'3 supported by thespaced floats 24' is attached to the vessel 20 by a spar'lfl whichholds'the forward endof cable 23 down at the depth desired-for theseis-'mometerfloats 25. Cable 23 preferably includes the insulated electricalconductors to 'the'varicus The depth-controlling floats 28 are similarlyspaced along a "separate tension cable TI likewise supported by spacedfioats 24 along its length. The trailing end of cable 23 may be attachedt'oaweight'12-supported from the buoyancy of the cable-supporting floats24 x on the cable'23- is regulated to provide .asneanly a neutralvalueas. possible whilethelseismometer plate spread -a nd auxiliary shootingvessel in 16 floats 25-. arejpositive, as before; Accordingly;

V .25 are slightly positively buoyant.

tension maintained inca ble .23 in towing the this, excess positivebuoyancy. of each of the. seismometer floats 25 is automatically.balanced by thedepth control cables 26 and 21.

V In Figures 8 and 9 are shown examples of singleunits of thedepth-controlled seismom- :eters in the relative 1130511710115 assumedby the .various elements when at equlibrium in the water. As in Figure8, the surface float 28 may be asingle, elongated structure fastened tothe ca- Me 23: andcarrying the depth control cables 26 and '21; or, itmay, as in Figure 9, comprise a .pair of floats 28a and 28b connected bythe flexible cable 23 and spaced slightly further apart :than the lengthof seismometer float 25, with each of the cables 26 and 21 attached toone of the pair.

jAs shown in Figure 10, the effectiveness of the cables 26 and 21 toregulate the depth of the float 25 may be increased and their range ofoperation narrowed-by attaching, by suitable qmeansat the bottom of theslack loop ofeach, c-anadditional elongated flexible weighting member16, such as a length of metal chain. With vthisarrangement, the positivebuoyancy of the kfloat 25 may be somewhat increased, and its range ofdepths at which stable equilibrium is achieved for a given buoyancyvariation isconsiderably narrowed.

: InFigure 11 is shown another arrangement of thesupporting float 28 andthe seismometer float, 25particularly adapted to the embodiment ofFigure 7. In this case, only a single weighted cable orlead 18 serves toregulatethe depth of ,the seism-ometer 25, which is maintained directlyunder the regulating float .28 by the cable '23, and1=the.,depth-regulating surface float 28 is towed by the separate cablell. vBy attaching the single depth-control lead 78 at a "central portionof the float 25 rather than at one end, a righting torque isexerted onthe float which will operate to maintain it in an upright position.

In many instances this will be sufficient to-permit v dispensing withthe trunnion mounting of seismometer 50, and the seismometer itself,made Water-tight, may be placed r inra-cavity within the float 25without any provision for rotation, thus simplifying its construction insome degree.

It is not essential'that the depth-controllead .18 be directly connectedto the seismometer float 25 andunder some conditions less noise will betransmitted tothe seismometer 50 by coupling the lead 18 to the cable 23or one of the cable floats 24 spaced laterally some distance away fromthe seismometer, as shown in Figure '12. In this arrangement weights 33are added to hring float 25 as exactly as possible to neutral buoyancy,and one or more of the cable'floats 24 on, either or bothsides of theseismometer float spread 22 through the water against the drag of float29 and-weight 12, automatically controlling the depth of laterallyspaced portions of the cable. 23 in this manner, effectively controlsthedep'th-of float 25 also. ,Such a control-arrangement can be appliedto as many or as few of the cable floats 24 as desired, or allof themcan be so controlled, and the slack loops of the control cables 18 willstill act to minimizesurface noise transmission.

A spread, so constructed that the points of attachment of thedepth-control leads to the submerged .cable are located at some distance:awayfrom the seismometers, maybe characterlzdtin'azgeneral .way bestating that the cable,

Due to the buoyancy, a plurality -of-.:spaced surface "floats,

seismometeraand submerged floats" have collectively asubstantiallyneutral buoyancy throughout the spread length except for a'plurality ofspaced points or small cable portions, each of which has a small netpositive. buoyancy due, .to I thesubmerged float attached at that point.This positive buoyancy at each :point orcable portion is countered bythe negative buoyancy of the attached depth-regulating lead whichextends to the surface float immediately above. It will further be notedthat the. cable 'll connecting "the surface floats, 28, as shown inFigures '7 and V :12, is not absolutely essential but maybe omittedrasolthat the surfacefloats 28 are towed along by'the submerged spreadpulling on thedepthcontrol cables Y26 and 21, or '18,,inthe samemanner-as the seismometer floats 25in Figure 1 are towed by thesecables. l 3 I While I have thus described my invention in terms of theforegoing specific embodiments,

numerous'useful modifications will occur to those skilled in the art.vFor example, many other arrangements of seismometer spacing and locationof the spread relative to the shot point are possible, and theartificial seismic waves may be generated in a variety of ways. Also thedepthcontrol leads 26 and 21 may be connected to surface floats at muchmore widely spaced points than those illustrated, in which case it isnot the total length of these leads. but thevertical projectionsof theirlengths which establish the depth of the seismometer float: That is, as'long .as the vertical projection of the portion; ofleads 2,6 and 21,regardlessof their actual length, sup

ported by the surface float or floats. 28 is 10 feet longer than thevertical projection-ofthat portion which the seismometer'float 25rcansupport,

the seismometerwill be held at aconstant depth or 10 feet; Where theseprojections are referred to in the claims,, they are called verticallengths. The scope of the invention therefore should not be consideredas limited to the exact details set forth but is to be ascertained-fromthe scope of the appended claims. I 1

I claim l. A marine .seismometer spread adapted for towing by a vesselthrough a body of ,Water comprising a tension and conductor cable, meanssupporting said-cable in the water, aplurality of 'seismometers spacedalong said cable,.'in-

sulated'electrical leads connecting said 'seismom eters to conductors insaid cable, positively buoy,- ant. float means coupled toreach of saidseismometers and forming-therewith spaced detector units each. having asmall positive resultant and a plurality of negatively buoyant flexibleleads, each of said detector units being separately connected to adifferent one of'said surface floats by atleast one of said flexibleleads .which forms V the sole mechanical linkage therebetween, the

length of eachlead being substantially greater than the desired depth ofsubmergence for said 2. A marine seismometer spread according to claim lin which there are twoof sa1d ne gat1ve 1y buoyant flexible leads whichform the sole mechanical linkages between said surface floats and eachof said detector units.

3. A marine seismometer spread according to claim 2 in which each ofsaid two flexible leads is coupled to a separate surface float.

4. A marine seismometer spread according to claim 1 in which the majorportion of the negative buoyancy of said lead is distributed along onlypart of its length close to the bottom of said slack loop.

5. A marine seismometer spread adapted for towing by a vessel through abody of water comprising a tension and conductor cable, means supportingsaid cable in the water, a plurality of seismometers spaced along andconnected to conductors in said cable, means supporting saidseismometers in the water, the buoyancy of said cable, seismometers, andsupporting means being adjusted to have a substantially neutral averagevalue when completely submerged in the water, except for a plurality ofportions spaced along said cable where the buoyancy has small positivevalues, and means for controlling the depth of each of said positivebuoyancy cable portions comprising a surface float and a negativelybuoyant flexible means forming the sole mechanical linkage between saidcable portion and said surface float, the length of said flexible meansbeing substantially greater than the desired depth of submergence forsaid cable portion and the negative buoyancy of a small portion of thelength of said flexible means being effective to balance said smallpositive buoyancy, and the flexibility of said flexible means being sogreat that said small positive buoyancy is able to flex said flexiblemeans until it hangs with a slack loop between its points of attachmentto said surface float and to said positive buoyancy cable portion,whereby said cable portions are held at positions of stable equilibriumbelow the surface of the water.

6. A marine seismometer spread adapted for towing by a vessel through abody of water comprising a first cable, means attached to said cablesupporting it at the surface of the water, a second cable having aplurality of insulated electrical conductors, a plurality ofseismometers spaced along said second cable and connected to saidconductors, means supporting said second cable and said seismometers inthe water and giving them a substantially neutral average buoyancytherein except for a plurality of small portions spaced along the lengthof said second cable, each of said portions having a small positivebuoyancy in the water, and a negatively buoyant flexible lead formingthe sole mechanical linkage between said first cable and each of saidsmall positive buoyancy portions of said second cable, the length ofsaid lead being substantially greater than the desired depth ofsubmergence for said seismometers and the negative buoyancy of a portionsubstantially less than half of the length of said lead being effectiveto balance said small positive buoyancy, the flexibility of said leadbeing so great that said small positive buoyancy is able to flex saidlead until it hangs with a slack loop between its points of attachmentto said first cable and to said small positive buoyancy portions,whereby said second cable and said seismometers are held at positions ofstable equilibrium below the surface of the water.

7, A marine seismometer spread adapted for towing by a vessel through abody of water comprising a tension and conductor cable, means supportingsaid cable at the surface of the water, and a plurality of units spacedalong the length of said cable, each of said units comprising a buoyantsurface float attached to said cable, a seismometer connected toconductors in said cable, a float supporting said seismometer and givingit a small positive buoyancy in the water, and a pair of negativelybuoyant flexible leads longer than the desired depth of submergence ofsaid seismometer forming the sole mechanical connections between saidsurface float and said seismometer-supporting float, the positivebuoyancy of said surface float being greater than the negative buoyancyof said connecting flexible leads, and the negative buoyancy of aportion of said leads being effective to balance said small positivebuoyancy, the flexibility of said leads being so great that said smallpositive buoyancy is able to flex said leads until they hang with slackloops between the points of attachment to said surface float and to saidseismometer-supporting float, whereby said seismometer is held at aposition of stable equilibrium below the surface of the water.

NEWELL POT'I'ORF.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,470,733 Hayes Oct. 16, 19232,203,894 Cook June 11, 1940 2,241,428 Silverman May 13, 1941 2,423,591Flude July 8, 1947 2,440,903 Massa May 4, 1948 2,449,085 Peterson Sept.14, 1948 2,465,696 Paslay Mar. 29, 1949 2,570,707 Parr, Jr. Oct. 9, 1951

